Why is Jesus Christ called the Son of God if he is actually not a real son?
As the Son of God, Jesus is like us in that he has a Father and we mere human beings have fathers as well. Jesus is infinitely different than us in that he is eternally begotten of his Father, whereas our human fathers precede us in existence. This is an aspect of the great mystery of the Trinity.
In addition, God has revealed himself as Father, Son, and Holy Spirit (see Matt. 28:18-20). God is “an eternal exchange of love” (Catechism 221) and has destined us to share in that exchange as his adopted sons and daughters (Catechism 1997).